Non-disruptive baseline and resynchronization of a synchronous replication relationship

ABSTRACT

One or more techniques and/or computing devices are provided for non-disruptively establishing a synchronous replication relationship between a primary volume and a secondary volume and/or for resynchronizing the primary volume and the secondary volume. For example, a baseline snapshot and one or more incremental snapshots of the primary volume are used to construct and incrementally update the secondary volume with data from the primary volume. A dirty region log is used to track modifications to the primary volume. A splitter object is used to split client write requests to the primary volume and to the secondary volume. A synchronous transfer engine session is initiated to processing incoming client write requests using the dirty region log. A cutover scanner is used to transfer dirty data from the primary volume to the secondary volume. In this way, a synchronous replication relationship is established between the primary volume and the secondary volume.

RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S.application Ser. No. 16/504,430, filed on Jul. 8, 2019, now allowed,titled “NON-DISRUPTIVE BASELINE AND RESYNCHRONIZATION OF A SYNCHRONOUSREPLICATION RELATIONSHIP,” which claims priority to and is acontinuation of U.S. Pat. No. 10,353,921, filed on Jul. 9, 2018, titled“NON-DISRUPTIVE BASELINE AND RESYNCHRONIZATION OF A SYNCHRONOUSREPLICATION RELATIONSHIP,” which claims priority to and is acontinuation of U.S. Pat. No. 10,019,502, filed on Nov. 27, 2015, titled“NON-DISRUPTIVE BASELINE AND RESYNCHRONIZATION OF A SYNCHRONOUSREPLICATION RELATIONSHIP,” which are incorporated herein by reference.

BACKGROUND

Many storage networks may implement data replication and/or otherredundancy data access techniques for data loss protection andnon-disruptive client access. For example, a first storage cluster maycomprise a first storage controller configured to provide clients withprimary access to data stored within a first storage device and/or otherstorage devices. A second storage cluster may comprise a second storagecontroller configured to provide clients with primary access to datastored within a second storage device and/or other storage devices. Thefirst storage controller and the second storage controller may beconfigured according to a disaster recovery relationship, such that thesecond storage controller may provide failover access to replicated datathat was replicated from the first storage device to a secondary storagedevice, owned by the first storage controller, but accessible to thesecond storage controller (e.g., a switchover operation may be performedwhere the second storage controller assumes ownership of the secondarystorage device and/or other storage devices previously owned by thefirst storage controller so that the second storage controller mayprovide clients with failover access to replicated data within suchstorage devices).

In an example, the second storage cluster may be located at a remotesite to the first storage cluster (e.g., storage clusters may be locatedin different buildings, cities, thousands of kilometers from oneanother, etc.). Thus, if a disaster occurs at a site of a storagecluster, then a surviving storage cluster may remain unaffected by thedisaster (e.g., a power outage of a building hosting the first storagecluster may not affect a second building hosting the second storagecluster in a different city).

In an example, two storage controllers within a storage cluster may beconfigured according to a high availability configuration, such as wherethe two storage controllers are locally connected to one another and/orto the same storage devices. In this way, when a storage controllerfails, then a high availability partner storage controller can quicklytakeover for the failed storage controller due to the localconnectivity. Thus, the high availability partner storage controller mayprovide clients with access to data previously accessible through thefailed storage controller.

Various replication and synchronization techniques may be used toreplicate data (e.g., client data), configuration data (e.g., a size ofa volume, a name of a volume, etc.), and/or write caching data (e.g.,cached write operations) between storage controllers and/or storagedevices. In an example, snapshots of a primary volume (e.g., within thefirst storage device) may be used to replicate the primary volume to asecondary volume (e.g., within the secondary storage device). Forexample, a base snapshot of the primary volume may be used to initiallycreate the secondary volume. A current incremental snapshot of theprimary volume may be used to replicate changes made to the primaryvolume since the base snapshot or since a last incremental snapshot.Unfortunately, synchronizing and/or resynchronizing the primary volumeand the secondary volume can be disruptive to client access to theprimary volume. For example, client write requests to the primary volumemay be rejected during a cutover phase of synchronization, thusincreasing latency and client data access disruption.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a component block diagram illustrating an example clusterednetwork in accordance with one or more of the provisions set forthherein.

FIG. 2 is a component block diagram illustrating an example data storagesystem in accordance with one or more of the provisions set forthherein.

FIG. 3 is a flow chart illustrating an exemplary method ofnon-disruptively establishing a synchronous replication relationship.

FIG. 4A is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where a baseline transfer is performed.

FIG. 4B is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where one or more incremental transfers are performed.

FIG. 4C is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where dirty region logs and splitter objects areestablished.

FIG. 4D is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where a client write request is processed.

FIG. 4E is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where an incoming client write request is processed.

FIG. 4F is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where an incoming client write request is processed.

FIG. 4G is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where a cutover scanner performs a dirty region transfer.

FIG. 4H is a component block diagram illustrating an exemplary computingdevice for non-disruptively establishing a synchronous replicationrelationship, where a synchronous replication relationship isestablished.

FIG. 5 is an example of a computer readable medium in accordance withone or more of the provisions set forth herein.

DETAILED DESCRIPTION

Some examples of the claimed subject matter are now described withreference to the drawings, where like reference numerals are generallyused to refer to like elements throughout. In the following description,for purposes of explanation, numerous specific details are set forth inorder to provide an understanding of the claimed subject matter. It maybe evident, however, that the claimed subject matter may be practicedwithout these specific details. Nothing in this detailed description isadmitted as prior art.

One or more techniques and/or computing devices for non-disruptivelyestablishing a synchronous replication relationship between a primaryvolume and a secondary volume and/or for resynchronizing the primaryvolume and the secondary volume are provided herein. For example, asynchronous replication relationship may be initially establishedbetween the primary volume (e.g., used to actively store client data foraccess) and the secondary volume (e.g., used as a backup to storereplicated client data from the primary volume) in a non-disruptivemanner with little to no client data access disruption to the primaryvolume. If the primary volume and the secondary volume become out ofsync over time (e.g., due to a network issue, a storage controllerfailure, etc.), then the primary volume and the secondary volume may beresynchronized in a non-disruptive manner.

To provide context for non-disruptively establishing and/orresynchronizing a synchronous replication relationship, FIG. 1illustrates an embodiment of a clustered network environment 100 or anetwork storage environment. It may be appreciated, however, that thetechniques, etc. described herein may be implemented within theclustered network environment 100, a non-cluster network environment,and/or a variety of other computing environments, such as a desktopcomputing environment. That is, the instant disclosure, including thescope of the appended claims, is not meant to be limited to the examplesprovided herein. It will be appreciated that where the same or similarcomponents, elements, features, items, modules, etc. are illustrated inlater figures but were previously discussed with regard to priorfigures, that a similar (e.g., redundant) discussion of the same may beomitted when describing the subsequent figures (e.g., for purposes ofsimplicity and ease of understanding).

FIG. 1 is a block diagram illustrating the clustered network environment100 that may implement at least some embodiments of the techniquesand/or systems described herein. The clustered network environment 100comprises data storage systems 102 and 104 that are coupled over acluster fabric 106, such as a computing network embodied as a privateInfiniband, Fibre Channel (FC), or Ethernet network facilitatingcommunication between the data storage systems 102 and 104 (and one ormore modules, component, etc. therein, such as, nodes 116 and 118, forexample). It will be appreciated that while two data storage systems 102and 104 and two nodes 116 and 118 are illustrated in FIG. 1, that anysuitable number of such components is contemplated. In an example, nodes116, 118 comprise storage controllers (e.g., node 116 may comprise aprimary or local storage controller and node 118 may comprise asecondary or remote storage controller) that provide client devices,such as host devices 108, 110, with access to data stored within datastorage devices 128, 130. Similarly, unless specifically providedotherwise herein, the same is true for other modules, elements,features, items, etc. referenced herein and/or illustrated in theaccompanying drawings. That is, a particular number of components,modules, elements, features, items, etc. disclosed herein is not meantto be interpreted in a limiting manner.

It will be further appreciated that clustered networks are not limitedto any particular geographic areas and can be clustered locally and/orremotely. Thus, in one embodiment a clustered network can be distributedover a plurality of storage systems and/or nodes located in a pluralityof geographic locations; while in another embodiment a clustered networkcan include data storage systems (e.g., 102, 104) residing in a samegeographic location (e.g., in a single onsite rack of data storagedevices).

In the illustrated example, one or more host devices 108, 110 which maycomprise, for example, client devices, personal computers (PCs),computing devices used for storage (e.g., storage servers), and othercomputers or peripheral devices (e.g., printers), are coupled to therespective data storage systems 102, 104 by storage network connections112, 114. Network connection may comprise a local area network (LAN) orwide area network (WAN), for example, that utilizes Network AttachedStorage (NAS) protocols, such as a Common Internet File System (CIFS)protocol or a Network File System (NFS) protocol to exchange datapackets. Illustratively, the host devices 108, 110 may begeneral-purpose computers running applications, and may interact withthe data storage systems 102, 104 using a client/server model forexchange of information. That is, the host device may request data fromthe data storage system (e.g., data on a storage device managed by anetwork storage control configured to process I/O commands issued by thehost device for the storage device), and the data storage system mayreturn results of the request to the host device via one or more storagenetwork connections 112, 114.

The nodes 116, 118 on clustered data storage systems 102, 104 cancomprise network or host nodes that are interconnected as a cluster toprovide data storage and management services, such as to an enterprisehaving remote locations, cloud storage (e.g., a storage endpoint may bestored within a data cloud), etc., for example. Such a node in theclustered network environment 100 can be a device attached to thenetwork as a connection point, redistribution point or communicationendpoint, for example. A node may be capable of sending, receiving,and/or forwarding information over a network communications channel, andcould comprise any device that meets any or all of these criteria. Oneexample of a node may be a data storage and management server attachedto a network, where the server can comprise a general purpose computeror a computing device particularly configured to operate as a server ina data storage and management system.

In an example, a first cluster of nodes such as the nodes 116, 118(e.g., a first set of storage controllers configured to provide accessto a first storage aggregate comprising a first logical grouping of oneor more storage devices) may be located on a first storage site. Asecond cluster of nodes, not illustrated, may be located at a secondstorage site (e.g., a second set of storage controllers configured toprovide access to a second storage aggregate comprising a second logicalgrouping of one or more storage devices). The first cluster of nodes andthe second cluster of nodes may be configured according to a disasterrecovery configuration where a surviving cluster of nodes providesswitchover access to storage devices of a disaster cluster of nodes inthe event a disaster occurs at a disaster storage site comprising thedisaster cluster of nodes (e.g., the first cluster of nodes providesclient devices with switchover data access to storage devices of thesecond storage aggregate in the event a disaster occurs at the secondstorage site).

As illustrated in the clustered network environment 100, nodes 116, 118can comprise various functional components that coordinate to providedistributed storage architecture for the cluster. For example, the nodescan comprise network modules 120, 122 and data modules 124, 126. Networkmodules 120, 122 can be configured to allow the nodes 116, 118 (e.g.,network storage controllers) to connect with host devices 108, 110 overthe storage network connections 112, 114, for example, allowing the hostdevices 108, 110 to access data stored in the distributed storagesystem. Further, the network modules 120, 122 can provide connectionswith one or more other components through the cluster fabric 106. Forexample, in FIG. 1, the network module 120 of node 116 can access asecond data storage device 130 by sending a request through the datamodule 126 of a second node 118.

Data modules 124, 126 can be configured to connect one or more datastorage devices 128, 130, such as disks or arrays of disks, flashmemory, or some other form of data storage, to the nodes 116, 118. Thenodes 116, 118 can be interconnected by the cluster fabric 106, forexample, allowing respective nodes in the cluster to access data on datastorage devices 128, 130 connected to different nodes in the cluster.Often, data modules 124, 126 communicate with the data storage devices128, 130 according to a storage area network (SAN) protocol, such asSmall Computer System Interface (SCSI) or Fiber Channel Protocol (FCP),for example. Thus, as seen from an operating system on nodes 116, 118,the data storage devices 128, 130 can appear as locally attached to theoperating system. In this manner, different nodes 116, 118, etc. mayaccess data blocks through the operating system, rather than expresslyrequesting abstract files.

It should be appreciated that, while the clustered network environment100 illustrates an equal number of network and data modules, otherembodiments may comprise a differing number of these modules. Forexample, there may be a plurality of network and data modulesinterconnected in a cluster that does not have a one-to-onecorrespondence between the network and data modules. That is, differentnodes can have a different number of network and data modules, and thesame node can have a different number of network modules than datamodules.

Further, a host device 108, 110 can be networked with the nodes 116, 118in the cluster, over the storage networking connections 112, 114. As anexample, respective host devices 108, 110 that are networked to acluster may request services (e.g., exchanging of information in theform of data packets) of nodes 116, 118 in the cluster, and the nodes116, 118 can return results of the requested services to the hostdevices 108, 110. In one embodiment, the host devices 108, 110 canexchange information with the network modules 120, 122 residing in thenodes 116, 118 (e.g., network hosts) in the data storage systems 102,104.

In one embodiment, the data storage devices 128, 130 comprise volumes132, which is an implementation of storage of information onto diskdrives or disk arrays or other storage (e.g., flash) as a file-systemfor data, for example. Volumes can span a portion of a disk, acollection of disks, or portions of disks, for example, and typicallydefine an overall logical arrangement of file storage on disk space inthe storage system. In one embodiment a volume can comprise stored dataas one or more files that reside in a hierarchical directory structurewithin the volume.

Volumes are typically configured in formats that may be associated withparticular storage systems, and respective volume formats typicallycomprise features that provide functionality to the volumes, such asproviding an ability for volumes to form clusters. For example, where afirst storage system may utilize a first format for their volumes, asecond storage system may utilize a second format for their volumes.

In the clustered network environment 100, the host devices 108, 110 canutilize the data storage systems 102, 104 to store and retrieve datafrom the volumes 132. In this embodiment, for example, the host device108 can send data packets to the network module 120 in the node 116within data storage system 102. The node 116 can forward the data to thedata storage device 128 using the data module 124, where the datastorage device 128 comprises volume 132A. In this way, in this example,the host device can access the volume 132A, to store and/or retrievedata, using the data storage system 102 connected by the networkconnection 112. Further, in this embodiment, the host device 110 canexchange data with the network module 122 in the host 118 within thedata storage system 104 (e.g., which may be remote from the data storagesystem 102). The host 118 can forward the data to the data storagedevice 130 using the data module 126, thereby accessing volume 1328associated with the data storage device 130.

It may be appreciated that non-disruptively establishing and/orresynchronizing a synchronous replication relationship may beimplemented within the clustered network environment 100. In an example,a synchronous replication relationship may be established between thevolume 132A of node 116 (e.g., a first storage controller) and thevolume 1328 of the node 118 (e.g., a second storage controller) in anon-disruptive manner with respect to client data access to the volume132A and/or the volume 1328. If the volume 132A and the volume 1328become out of sync, then the volume 132A and the volume 1328 may beresynchronized in a non-disruptive manner. It may be appreciated thatnon-disruptively establishing and/or resynchronizing a synchronousreplication relationship may be implemented for and/or between any typeof computing environment, and may be transferrable between physicaldevices (e.g., node 116, node 118, etc.) and/or a cloud computingenvironment (e.g., remote to the clustered network environment 100).

FIG. 2 is an illustrative example of a data storage system 200 (e.g.,102, 104 in FIG. 1), providing further detail of an embodiment ofcomponents that may implement one or more of the techniques and/orsystems described herein. The data storage system 200 comprises a node202 (e.g., host nodes 116, 118 in FIG. 1), and a data storage device 234(e.g., data storage devices 128, 130 in FIG. 1). The node 202 may be ageneral purpose computer, for example, or some other computing deviceparticularly configured to operate as a storage server. A host device205 (e.g., 108, 110 in FIG. 1) can be connected to the node 202 over anetwork 216, for example, to provides access to files and/or other datastored on the data storage device 234. In an example, the node 202comprises a storage controller that provides client devices, such as thehost device 205, with access to data stored within data storage device234.

The data storage device 234 can comprise mass storage devices, such asdisks 224, 226, 228 of a disk array 218, 220, 222. It will beappreciated that the techniques and systems, described herein, are notlimited by the example embodiment. For example, disks 224, 226, 228 maycomprise any type of mass storage devices, including but not limited tomagnetic disk drives, flash memory, and any other similar media adaptedto store information, including, for example, data (D) and/or parity (P)information.

The node 202 comprises one or more processors 204, a memory 206, anetwork adapter 210, a cluster access adapter 212, and a storage adapter214 interconnected by a system bus 242. The data storage system 200 alsoincludes an operating system 208 installed in the memory 206 of the node202 that can, for example, implement a Redundant Array of Independent(or Inexpensive) Disks (RAID) optimization technique to optimize areconstruction process of data of a failed disk in an array.

The operating system 208 can also manage communications for the datastorage system, and communications between other data storage systemsthat may be in a clustered network, such as attached to a cluster fabric215 (e.g., 106 in FIG. 1). Thus, the node 202, such as a network storagecontroller, can respond to host device requests to manage data on thedata storage device 234 (e.g., or additional clustered devices) inaccordance with these host device requests. The operating system 208 canoften establish one or more file systems on the data storage system 200,where a file system can include software code and data structures thatimplement a persistent hierarchical namespace of files and directories,for example. As an example, when a new data storage device (not shown)is added to a clustered network system, the operating system 208 isinformed where, in an existing directory tree, new files associated withthe new data storage device are to be stored. This is often referred toas “mounting” a file system.

In the example data storage system 200, memory 206 can include storagelocations that are addressable by the processors 204 and networkadapters 210, 212, 214 for storing related software application code anddata structures. The processors 204 and network adapters 210, 212, 214may, for example, include processing elements and/or logic circuitryconfigured to execute the software code and manipulate the datastructures. The operating system 208, portions of which are typicallyresident in the memory 206 and executed by the processing elements,functionally organizes the storage system by, among other things,invoking storage operations in support of a file service implemented bythe storage system. It will be apparent to those skilled in the art thatother processing and memory mechanisms, including various computerreadable media, may be used for storing and/or executing applicationinstructions pertaining to the techniques described herein. For example,the operating system can also utilize one or more control files (notshown) to aid in the provisioning of virtual machines.

The network adapter 210 includes the mechanical, electrical andsignaling circuitry needed to connect the data storage system 200 to ahost device 205 over a network 216, which may comprise, among otherthings, a point-to-point connection or a shared medium, such as a localarea network. The host device 205 (e.g., 108, 110 of FIG. 1) may be ageneral-purpose computer configured to execute applications. Asdescribed above, the host device 205 may interact with the data storagesystem 200 in accordance with a client/host model of informationdelivery.

The storage adapter 214 cooperates with the operating system 208executing on the node 202 to access information requested by the hostdevice 205 (e.g., access data on a storage device managed by a networkstorage controller). The information may be stored on any type ofattached array of writeable media such as magnetic disk drives, flashmemory, and/or any other similar media adapted to store information. Inthe example data storage system 200, the information can be stored indata blocks on the disks 224, 226, 228. The storage adapter 214 caninclude input/output (I/O) interface circuitry that couples to the disksover an I/O interconnect arrangement, such as a storage area network(SAN) protocol (e.g., Small Computer System Interface (SCSI), iSCSI,hyperSCSI, Fiber Channel Protocol (FCP)). The information is retrievedby the storage adapter 214 and, if necessary, processed by the one ormore processors 204 (or the storage adapter 214 itself) prior to beingforwarded over the system bus 242 to the network adapter 210 (and/or thecluster access adapter 212 if sending to another node in the cluster)where the information is formatted into a data packet and returned tothe host device 205 over the network 216 (and/or returned to anothernode attached to the cluster over the cluster fabric 215).

In one embodiment, storage of information on disk arrays 218, 220, 222can be implemented as one or more storage volumes 230, 232 that arecomprised of a cluster of disks 224, 226, 228 defining an overalllogical arrangement of disk space. The disks 224, 226, 228 that compriseone or more volumes are typically organized as one or more groups ofRAIDs. As an example, volume 230 comprises an aggregate of disk arrays218 and 220, which comprise the cluster of disks 224 and 226.

In one embodiment, to facilitate access to disks 224, 226, 228, theoperating system 208 may implement a file system (e.g., write anywherefile system) that logically organizes the information as a hierarchicalstructure of directories and files on the disks. In this embodiment,respective files may be implemented as a set of disk blocks configuredto store information, whereas directories may be implemented asspecially formatted files in which information about other files anddirectories are stored.

Whatever the underlying physical configuration within this data storagesystem 200, data can be stored as files within physical and/or virtualvolumes, which can be associated with respective volume identifiers,such as file system identifiers (FSIDs), which can be 32-bits in lengthin one example.

A physical volume corresponds to at least a portion of physical storagedevices whose address, addressable space, location, etc. doesn't change,such as at least some of one or more data storage devices 234 (e.g., aRedundant Array of Independent (or Inexpensive) Disks (RAID system)).Typically the location of the physical volume doesn't change in that the(range of) address(es) used to access it generally remains constant.

A virtual volume, in contrast, is stored over an aggregate of disparateportions of different physical storage devices. The virtual volume maybe a collection of different available portions of different physicalstorage device locations, such as some available space from each of thedisks 224, 226, and/or 228. It will be appreciated that since a virtualvolume is not “tied” to any one particular storage device, a virtualvolume can be said to include a layer of abstraction or virtualization,which allows it to be resized and/or flexible in some regards.

Further, a virtual volume can include one or more logical unit numbers(LUNs) 238, directories 236, Qtrees 235, and files 240. Among otherthings, these features, but more particularly LUNS, allow the disparatememory locations within which data is stored to be identified, forexample, and grouped as data storage unit. As such, the LUNs 238 may becharacterized as constituting a virtual disk or drive upon which datawithin the virtual volume is stored within the aggregate. For example,LUNs are often referred to as virtual drives, such that they emulate ahard drive from a general purpose computer, while they actually comprisedata blocks stored in various parts of a volume.

In one embodiment, one or more data storage devices 234 can have one ormore physical ports, wherein each physical port can be assigned a targetaddress (e.g., SCSI target address). To represent respective volumesstored on a data storage device, a target address on the data storagedevice can be used to identify one or more LUNs 238. Thus, for example,when the node 202 connects to a volume 230, 232 through the storageadapter 214, a connection between the node 202 and the one or more LUNs238 underlying the volume is created.

In one embodiment, respective target addresses can identify multipleLUNs, such that a target address can represent multiple volumes. The I/Ointerface, which can be implemented as circuitry and/or software in thestorage adapter 214 or as executable code residing in memory 206 andexecuted by the processors 204, for example, can connect to volume 230by using one or more addresses that identify the one or more LUNs 238.

It may be appreciated that non-disruptively establishing and/orresynchronizing a synchronous replication relationship may beimplemented for the data storage system 200. In an example, asynchronous replication relationship may be established between thevolume 230 of the node 202 (e.g., a first storage controller) and asecond volume of a second node (e.g., a second storage controller) in anon-disruptive manner with respect to client data access to the volume230. If the volume 230 and the second volume become out of sync, thenthe volume 230 and the second volume may be resynchronized in anon-disruptive manner. It may be appreciated that non-disruptivelyestablishing and/or resynchronizing a synchronous replicationrelationship may be implemented for and/or between any type of computingenvironment, and may be transferrable between physical devices (e.g.,node 202, host device 205, etc.) and/or a cloud computing environment(e.g., remote to the node 202 and/or the host device 205).

One embodiment of non-disruptively establishing and/or resynchronizing asynchronous replication relationship is illustrated by an exemplarymethod 300 of FIG. 3. In an example, a synchronous replicationrelationship may be established between a first storage controller,hosting a primary volume, and a second storage controller. It may beappreciated that the synchronous replication relationship may beestablished for a file within the primary volume, a LUN within theprimary volume, a consistency group of one or more files or LUNs, aconsistency group spanning any number of primary volumes, a subdirectorywithin the primary volume, and/or any other grouping of data, and thatthe techniques described herein are not limited to merely a singleprimary volume and secondary volume, but can apply to any number offiles, LUNs, volumes, and/or consistency groups. The synchronousreplication relationship may be established in a non-disruptive mannersuch that client access to the primary volume may be facilitated duringthe establishment of the synchronous replication relationship.Accordingly, a base snapshot, of the primary volume, may be created. Thebase snapshot may comprise a point in time representation of data withinthe primary volume, such as data within a consistency group of filesand/or storage objects. At 302, a baseline transfer of data from theprimary volume to the second storage controller may be performed usingthe base snapshot to create a secondary volume accessible to the secondstorage controller.

At 304, one or more incremental transfers may be performed between theprimary volume and the secondary volume until a synchronization criteriais met (e.g., and/or other primary volumes and/or secondary volumeswhere the synchronous replication relationship exists for a consistencygroup spanning multiple primary volumes). For example, thesynchronization criteria may correspond to a threshold number ofincremental transfers or where a last incremental transfer transfers anamount of data below a threshold (e.g., about 10 mb or any other valueindicative of the primary volume and the secondary volume having arelatively small amount of divergence). In an example, an incrementalsnapshot of the primary volume may be created. The incremental snapshotmay correspond to a point in time representation of data within theprimary snapshot at a time subsequent to when the base snapshot wascreated. A difference between the incremental snapshot and a priorsnapshot (e.g., a snapshot used to perform the baseline transfer or alast incremental transfer) of the primary volume may be used to performan incremental transfer of data (e.g., differences in data within theprimary volume from when the prior snapshot was created and when theincremental snapshot was created). For example, a block levelincremental transfer, of data blocks that are different between theprior snapshot and the incremental snapshot, may be performed.Responsive to completion of the incremental transfer, a common snapshotmay be created from the secondary volume. For example, the commonsnapshot may be used to roll the secondary volume back to a state whenthe secondary volume mirrored the primary volume, such as for performinga resynchronization between the primary volume and the secondary volume.

At 306, dirty region logs may be initialized (e.g., in memory) to trackmodifications of files or LUNs within the primary volume (e.g., and/orother primary volumes where the synchronous replication relationshipexists for a consistency group spanning multiple primary volumes) (e.g.,track dirty data that has been written to the primary volume but not yetreplicated to the secondary volume). For example, a dirty region log fora file or LUN may comprise bits that can be set to indicate whetherregions of the file have been modified by client write requests thathave not yet been replicated to the secondary volume. For example, aclient write request, targeting a region of the file or LUNs, may bereceived. The client write request may be implemented upon the region(e.g., to write client data into the region). Responsive to successfulimplementation of the client write request, a bit within the dirtyregion log may be set to indicate that the region is a dirty regioncomprising dirty data. The bit may be reset once the client writerequest and/or the dirty data has been successfully replicated to thesecondary volume.

At 308, splitter objects for end points, such as files or LUNs, may beconfigured to subsequently split client write requests (e.g., at 312) tothe primary volume (e.g., and/or other primary volumes and/or secondaryvolumes where the synchronous replication relationship exists for aconsistency group spanning multiple primary volumes) and to thesecondary volume (e.g., a splitter object is created and associated witha dirty region log for a file or LUN). For example, a splitter objectmay be subsequently used to intercept and split a client write request(e.g., before the client write request is received by a file system)into a replication client write request so that the client write requestcan be locally implemented by the first storage controller upon theprimary volume and the replication client write request can be remotelyimplemented by the second storage controller upon the secondary volume.At this point, the splitter object starts tracking dirty regions usingthe dirty region logs.

At 310, responsive to the dirty region logs tracking modifications tothe primary volume (e.g., marking regions, modified by client writerequests, as dirty), an asynchronous transfer from the primary volume tothe secondary volume may be performed (e.g., a final incrementaltransfer).

At 312, a synchronous transfer engine session may be initiated betweenthe primary volume and the secondary volume (e.g., and/or other primaryvolumes and/or secondary volumes where the synchronous replicationrelationship exists for a consistency group spanning multiple primaryvolumes), such that a transfer engine is replicating incoming clientwrite requests to the secondary volume based upon data within the dirtyregion log. For example, responsive to an incoming client write requesttargeting a dirty region of the file or LUN within the primary volume(e.g., a bit within the dirty region log may indicate that the dirtyregion has been modified and that the modification has not yet beenreplicated to the secondary volume), the incoming client write requestmay be committed to the primary volume and not split for replication tothe secondary volume because the dirty region will be subsequentlyreplicated to the secondary volume by a cutover scanner. Responsive tothe incoming client write request corresponding to a non-dirty region,the incoming client write request may be locally committed to thenon-dirty region of the primary volume and a replication client writerequest, split from the incoming client write request, may be remotelycommitted to the secondary volume. Responsive to the incoming clientwrite request corresponding to a partially dirty region associated withan overlap between a dirty block and a non-dirty block, the incomingclient write request may be locally committed to the partially dirtyregion of the primary volume (e.g., committed to the dirty and non-dirtyblocks) and the entire replication client write request may be remotelycommitted to the secondary volume.

At 314, the cutover scanner may be initiated to scan the dirty regionlog for transferring dirty data of dirty regions from the primary volumeto the secondary volume (e.g., and/or other primary volumes and/orsecondary volumes where the synchronous replication relationship existsfor a consistency group spanning multiple primary volumes). For example,the cutover scanner may identify a current dirty region of the primaryvolume using the dirty region log. A lock may be set for the currentdirty region to block incoming client write requests to the currentdirty region. In an example, while the lock is set, a new incomingclient write request, targeting the current dirty region, may be queued.Dirty data of the current dirty region may be transferred to the secondstorage controller for storage into the secondary volume. The bit,within the dirty region log, may be reset to indicate that the currentdirty region is now a clean region with clean data replicated to thesecondary volume. Responsive to successful storage of the dirty datainto the secondary volume and/or the bit being reset, the current dirtyregion may be unlocked. Responsive to the current dirty region (e.g.,the clean region) being unlocked, the new incoming client write requestmay be processed (e.g., the clean region may be locked while the newincoming client write request is being implemented upon the cleanregion, and then the clean region may be unlocked).

Responsive to the cutover scanner completing, the primary volume and thesecondary volume may be designated as being in the synchronousreplication relationship. While in the synchronous replicationrelationship, a current client write request to the primary volume maybe received. The current client write request may be split into acurrent replication client write request. The current client writerequest may be locally committed to the primary storage. The currentreplication write request may be sent to the second storage controllerfor remote commitment to the secondary volume. Responsive to the currentclient write request being locally committed and the current replicationclient write request being remotely committed, a completion notificationmay be sent to a client that submitted the current client write request.

In an example, the primary volume and the secondary volume may becomeout of sync for various reasons, such as network issues, a storagecontroller failure, etc. Accordingly, a common snapshot between theprimary volume and the secondary volume may be used to roll thesecondary volume back to a state where the secondary volume mirrored theprimary volume. The synchronous replication relationship may bereestablished in a non-disruptive manner (e.g., the primary volume mayremain accessible to clients during the resynchronization). For example,the dirty region logs, the splitter objects, the synchronous transferengine session, and/or the cutover scanner may be used to reestablishthe synchronous replication relationship (e.g., at least some of theactions 302, 304, 306, 308, 310, 312, and/or 314 may be performed toreestablish the synchronous replication relationship in a non-disruptivemanner).

In an example, the dirty region logs, the splitter objects, thesynchronous transfer engine session, and/or the cutover scanner may beused to perform a volume migration operation of the primary volume. Forexample, the primary volume may be migrated in a non-disruptive mannerwhere a relatively smaller disruption interval is achieved. In this way,client access may be facilitated to the primary volume during the volumemigration operation.

In an example, a flip resync may be performed in response to aswitchover operation from the first storage controller to the secondstorage controller (e.g., the first storage controller may fail, andthus the second storage controller may take ownership of storage devicespreviously owned by the first storage controller, such as a storagedevice hosting the secondary volume, so that the second storagecontroller may provide clients with failover access to replicated datafrom the storage devices such as to the secondary volume). Accordingly,the techniques described in relation to method 300 may be implemented toperform the flip resync to synchronizing data from the secondary volume(e.g., now actively used as a primary by the second storage controllerto provide clients with failover access to data) to the primary volume(e.g., now used as a secondary backup volume during the switchoveroperation).

FIGS. 4A-4H illustrate examples of a system for non-disruptivelyestablishing and/or resynchronizing a synchronous replicationrelationship. FIG. 4A illustrates a first storage controller 402 and asecond storage controller 404 having connectivity over a network 406(e.g., the storage controllers may reside in the same or differentclusters). The source storage controller 402 may comprise a primaryvolume 408 for which a synchronous replication relationship is to beestablished with the second storage controller 404. Accordingly, a basesnapshot 410 of the primary volume 408 or portion thereof (e.g., asnapshot of a consistency group, such as a grouping of files or storageobjects) may be created. A baseline transfer 412, using the basesnapshot 410, may be performed to transfer data from the primary volume408 to the second storage controller 404 for creating a secondary volume414, such that the secondary volume 414 is populated with data mirroringthe primary volume 408 as represented by the base snapshot 410.

FIG. 4B illustrates one or more incremental transfers being performedfrom the first storage controller 402 to the second storage controller404. For example, an incremental snapshot 420 of the primary volume 408may be created. The incremental snapshot 420 may comprise a point intime representation of the primary volume 408 or portion thereof at asubsequent time from when the base snapshot 410 was created. Theincremental snapshot 420 and the base snapshot 410 (e.g., or a lastincremental snapshot used to perform a most recent incremental transfer)may be compared to identify differences of the primary volume 408 fromwhen the incremental snapshot 420 was created and when last snapshot,such as the base snapshot 410, was created and transferred/replicated tothe secondary volume 414 (e.g., files, directories, and/or hard linksmay be created, deleted, and/or moved within the primary volume 408,thus causing a divergence between the primary volume 408 and thesecondary volume 414). In an example, data differences may betransferred using the incremental transfer 422. In another example,storage operations, corresponding to the differences (e.g., a create newfile operation, a delete file operation, a move file operation, a createnew directory operation, a delete directory operation, a move directoryoperation, and/or other operations that may be used by the secondstorage controller 404 to modify the secondary volume 414 to mirror theprimary volume 408 as represented by the incremental snapshot 420), maybe transferred to the second storage controller 404 using theincremental transfer 422 for implementation upon the secondary volume414. In this way, files, directories, hard links, and/or data within thesecondary volume 414 may mirror the primary volume 408 as represented bythe incremental snapshot 420. Incremental transfers, using incrementalsnapshots, may be performed until a synchronization criteria is met(e.g., a threshold number of incremental transfers, a last transfertransferring an amount of data below a threshold, etc.).

FIG. 4C illustrates dirty region logs 430 being initialized for trackingmodifications of files or LUNs within the primary volume 408. Forexample, a dirty region log may comprise bits that may be set toindicate whether regions of a file or LUN have been modified by clientwrite requests that have not been replicated to the secondary volume 414and thus are dirty regions, or whether regions are synchronized with thesame data between the primary volume 408 and the secondary volume 414and thus are clean regions. Splitter objects 431, for endpoints such asthe second storage controller 404 or other storage controllers, may beconfigured to split client write requests to the primary volume 408 andto the secondary volume 414. Responsive to the dirty region logs 430tracking modifications, an asynchronous transfer 433 of data from theprimary volume 408 to the secondary volume 414 may be performed (e.g., afinal incremental transfer).

FIG. 4D illustrates the dirty region logs 430 being used to trackmodifications by client write requests to the primary volume 408. Forexample, the first storage controller 402 may receive a client writerequest 434 targeting a second region within a file ABC. The clientwrite request 434 may be locally implemented 436 upon the primary volume408. Accordingly, a bit, corresponding to the second region of the fileABC, may be set to indicate that the second region is a dirty regionbecause the modification of the client write request 434 has not yetbeen replicated to the secondary volume 414.

FIG. 4E illustrates the splitter objects 431 performing client writerequest splitting. In an example, a synchronous transfer engine sessionmay be initiated to use the dirty region logs 430 and/or the splitterobjects 431 to process incoming client write requests. For example, anincoming client write request 442 may be received by the first storagecontroller 402. The dirty region logs 430 may be evaluated to determinethat the incoming client write request 442 targets a non-dirty regionwithin the primary volume 408. Accordingly, the incoming client writerequest 442 may be locally implemented 444 upon the primary volume 408.The incoming client write request 442 may be split by the splitterobjects 431 into a replication client write request 446 that is sent tothe second storage controller 404 for remote implementation 448 upon thesecondary volume 414. In another example, a second incoming client writerequest, not illustrated, may be received by the first storagecontroller 402. The second incoming client write request may correspondto a partially dirty region that is associated with an overlap betweenone or more dirty blocks and one or more non-dirty blocks of the primaryvolume 408. Accordingly, the first storage controller 402 may locallycommit the entire second incoming client write request to the primaryvolume 408, and the entire second incoming client write request may bereplicated to the secondary volume 414.

FIG. 4F illustrates the splitter objects 431 performing client writerequest splitting. For example, an incoming client write request 450 maybe received by the first storage controller 402. The dirty region logs430 may be evaluated to determine that the incoming client write request450 targets a dirty region within the primary volume 408. Accordingly,the incoming client write request 450 may be locally implemented 452upon the primary volume 408, but not replicated to the secondary volume414 because a cutover scanner may subsequently replicated dirty datawithin the dirty region to the secondary volume 414.

FIG. 4G illustrates the cutover scanner 460 being initiated to scan thedirty region logs 430 for transferring dirty data of dirty regions fromthe primary volume 408 to the secondary volume 414. For example, thecutover scanner 460 may scan the dirty region logs 430 to determine thatthe second region of file ABC is a dirty region. Accordingly, dirty datawithin the dirty region is replicated to the secondary volume 414 usinga dirty region transfer 462, and the dirty region logs 430 are modifiedto indicate that the second region is now clean. In this way, thecutover scanner 460 replicates dirty data to the secondary volume 414 sothat the secondary volume 414 mirrors the primary volume 408 (e.g., toreduce or eliminate data divergence between the primary volume 408 andthe secondary volume 414 in order to bring the primary volume 408 andthe secondary volume 414 into sync). Additionally, the splitter objects431 are splitting and replicating incoming client write requests to theprimary volume 408 and the secondary volume 414, which can also reduceor eliminate data divergence in order to bring the primary volume 408and the secondary volume 414 into sync. Thus, once the cutover scanner460 is complete, the primary volume 408 and the secondary volume 414 aredesignated as being in a synchronous replication relationship 470 wheredata consistency is maintained between the primary volume 408 and thesecondary volume 414 (e.g., client write requests are committed to boththe primary volume 408 and the secondary volume 414 before clientrequests are responded back to clients as being complete), asillustrated in FIG. 4H.

In an example, the primary volume 408 and the secondary volume 414 maybecome out of sync for various reasons, such as network issues, astorage controller failure, etc. Accordingly, a common snapshot betweenthe primary volume 408 and the secondary volume 414 may be used to rollthe secondary volume 414 back to a state where the secondary volume 414mirrored the primary volume 408. Once the secondary volume 414 has beenrolled back, the synchronous replication relationship 470 may bereestablished using the techniques described above (e.g., method 300 ofFIG. 3 and/or FIGS. 4A-4G) that were used to initially establish thesynchronous replication relationship 470. For example, the dirty regionlogs 430, the splitter objects 431, the synchronous transfer enginesession, and/or the cutover scanner 460 may be used to reestablish thesynchronous replication relationship 470.

In an example, the dirty region logs 430, the splitter objects 431, thesynchronous transfer engine session, and/or the cutover scanner 460 maybe used to perform a volume migration operation of the primary volume408. For example, the primary volume 408 may be migrated in anon-disruptive manner where a relatively smaller disruption interval isachieved. In this way, client access may be facilitated to the primaryvolume 408 during the volume migration operations.

Still another embodiment involves a computer-readable medium comprisingprocessor-executable instructions configured to implement one or more ofthe techniques presented herein. An example embodiment of acomputer-readable medium or a computer-readable device that is devisedin these ways is illustrated in FIG. 5, wherein the implementation 500comprises a computer-readable medium 508, such as a CD-ft DVD-R, flashdrive, a platter of a hard disk drive, etc., on which is encodedcomputer-readable data 506. This computer-readable data 506, such asbinary data comprising at least one of a zero or a one, in turncomprises a processor-executable computer instructions 504 configured tooperate according to one or more of the principles set forth herein. Insome embodiments, the processor-executable computer instructions 504 areconfigured to perform a method 502, such as at least some of theexemplary method 300 of FIG. 3, for example. In some embodiments, theprocessor-executable computer instructions 504 are configured toimplement a system, such as at least some of the exemplary system 400 ofFIGS. 4A-4H, for example. Many such computer-readable media arecontemplated to operate in accordance with the techniques presentedherein.

It will be appreciated that processes, architectures and/or proceduresdescribed herein can be implemented in hardware, firmware and/orsoftware. It will also be appreciated that the provisions set forthherein may apply to any type of special-purpose computer (e.g., filehost, storage server and/or storage serving appliance) and/orgeneral-purpose computer, including a standalone computer or portionthereof, embodied as or including a storage system. Moreover, theteachings herein can be configured to a variety of storage systemarchitectures including, but not limited to, a network-attached storageenvironment and/or a storage area network and disk assembly directlyattached to a client or host computer. Storage system should thereforebe taken broadly to include such arrangements in addition to anysubsystems configured to perform a storage function and associated withother equipment or systems.

In some embodiments, methods described and/or illustrated in thisdisclosure may be realized in whole or in part on computer-readablemedia. Computer readable media can include processor-executableinstructions configured to implement one or more of the methodspresented herein, and may include any mechanism for storing this datathat can be thereafter read by a computer system. Examples of computerreadable media include (hard) drives (e.g., accessible via networkattached storage (NAS)), Storage Area Networks (SAN), volatile andnon-volatile memory, such as read-only memory (ROM), random-accessmemory (RAM), EEPROM and/or flash memory, CD-ROMs, CD-Rs, CD-RWs, DVDs,cassettes, magnetic tape, magnetic disk storage, optical or non-opticaldata storage devices and/or any other medium which can be used to storedata.

Although the subject matter has been described in language specific tostructural features or methodological acts, it is to be understood thatthe subject matter defined in the appended claims is not necessarilylimited to the specific features or acts described above. Rather, thespecific features and acts described above are disclosed as exampleforms of implementing at least some of the claims.

Various operations of embodiments are provided herein. The order inwhich some or all of the operations are described should not beconstrued to imply that these operations are necessarily orderdependent. Alternative ordering will be appreciated given the benefit ofthis description. Further, it will be understood that not all operationsare necessarily present in each embodiment provided herein. Also, itwill be understood that not all operations are necessary in someembodiments.

Furthermore, the claimed subject matter is implemented as a method,apparatus, or article of manufacture using standard application orengineering techniques to produce software, firmware, hardware, or anycombination thereof to control a computer to implement the disclosedsubject matter. The term “article of manufacture” as used herein isintended to encompass a computer application accessible from anycomputer-readable device, carrier, or media. Of course, manymodifications may be made to this configuration without departing fromthe scope or spirit of the claimed subject matter.

As used in this application, the terms “component”, “module,” “system”,“interface”, and the like are generally intended to refer to acomputer-related entity, either hardware, a combination of hardware andsoftware, software, or software in execution. For example, a componentincludes a process running on a processor, a processor, an object, anexecutable, a thread of execution, an application, or a computer. By wayof illustration, both an application running on a controller and thecontroller can be a component. One or more components residing within aprocess or thread of execution and a component may be localized on onecomputer or distributed between two or more computers.

Moreover, “exemplary” is used herein to mean serving as an example,instance, illustration, etc., and not necessarily as advantageous. Asused in this application, “or” is intended to mean an inclusive “or”rather than an exclusive “or”. In addition, “a” and “an” as used in thisapplication are generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Also, at least one of A and B and/or the like generally means A orB and/or both A and B. Furthermore, to the extent that “includes”,“having”, “has”, “with”, or variants thereof are used, such terms areintended to be inclusive in a manner similar to the term “comprising”.

Many modifications may be made to the instant disclosure withoutdeparting from the scope or spirit of the claimed subject matter. Unlessspecified otherwise, “first,” “second,” or the like are not intended toimply a temporal aspect, a spatial aspect, an ordering, etc. Rather,such terms are merely used as identifiers, names, etc. for features,elements, items, etc. For example, a first set of information and asecond set of information generally correspond to set of information Aand set of information B or two different or two identical sets ofinformation or the same set of information.

Also, although the disclosure has been shown and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others skilled in the art based upon a reading andunderstanding of this specification and the annexed drawings. Thedisclosure includes all such modifications and alterations and islimited only by the scope of the following claims. In particular regardto the various functions performed by the above described components(e.g., elements, resources, etc.), the terms used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure. In addition, while aparticular feature of the disclosure may have been disclosed withrespect to only one of several implementations, such feature may becombined with one or more other features of the other implementations asmay be desired and advantageous for any given or particular application.

1. A method comprising: in response to receiving a write request from afirst client device when a first consistency group hosted by a firstnode and a second consistency group hosted by a second node are out ofsync: executing the write request upon the first consistency group; andmarking, within a dirty region log, a region modified by the writerequest as a dirty region; and performing a non-disruptiveresynchronization to synchronize the second consistency group and thefirst consistency group, wherein the non-disruptive resynchronizationincludes: committing incoming write requests corresponding to the dirtyregions to the first consistency group and not the second consistencygroup and committing incoming write requests corresponding to non-dirtyregions to the first consistency group and the second consistency groupto place the first consistency group and second consistency group into asynchronous replication state.
 2. The method of claim 1, comprising:performing an asynchronous transfer of data from the first consistencygroup to the second consistency group based upon the dirty region logbeing used to track the dirty regions modified by write requests.
 3. Themethod of claim 1, comprising: providing, by the second node, a secondclient device with access to data hosted by the second node while thefirst node provides the first client device with access to the firstconsistency group.
 4. The method of claim 1, wherein the performing thenon-disruptive resynchronization comprises: in response to performing athreshold number of asynchronous transfers from the first consistencygroup to the second consistency group, transferring data of the firstconsistency group to the second consistency group to synchronize thesecond consistency group and the first consistency group.
 5. The methodof claim 1, comprising: providing, by the first node, the first clientdevice with access to the first consistency group during thenon-disruptive resynchronization.
 6. The method of claim 1, comprising:providing, by the second node, a second client device with access todata hosted by the second node during the non-disruptiveresynchronization.
 7. The method of claim 1, comprising: performing abaseline transfer using a base snapshot to transfer data of the firstconsistency group to the second node to create the second consistencygroup.
 8. The method of claim 1, comprising: providing the first clientdevice with access to the first consistency group while performing abaseline transfer using a base snapshot to transfer data of the firstconsistency group to the second node to create the second consistencygroup.
 9. A non-transitory machine readable medium having stored thereoninstructions, which when executed by a machine, causes the machine to:in response to receiving a write request from a first client device whena first consistency group hosted by a first node and a secondconsistency group hosted by a second node are out of sync: execute thewrite request upon the first consistency group; and mark, within a dirtyregion log, a region modified by the write request as a dirty region;and perform a non-disruptive resynchronization to synchronize the secondconsistency group and the first consistency group, wherein thenon-disruptive resynchronization includes: committing incoming writerequests corresponding to the dirty regions to the first consistencygroup and not the second consistency group and committing incoming writerequests corresponding to non-dirty regions to the first consistencygroup and the second consistency group to place the first consistencygroup and second consistency group into a synchronous replication state.10. The non-transitory machine readable medium of claim 9, wherein theinstructions cause the machine to: perform the non-disruptiveresynchronization in response to a communication loss.
 11. Thenon-transitory machine readable medium of claim 9, wherein theinstructions cause the machine to: provide, by the second node, a secondclient device with access to data hosted by the second node while thefirst node provides the first client device with access to the firstconsistency group.
 12. The non-transitory machine readable medium ofclaim 9, wherein the instructions cause the machine to: in response toperforming a threshold number of asynchronous transfers from the firstconsistency group to the second consistency group, transfer data of thefirst consistency group to the second consistency group to synchronizethe second consistency group and the first consistency group.
 13. Thenon-transitory machine readable medium of claim 9, wherein theinstructions cause the machine to: provide, by the first node, the firstclient device with access to the first consistency group during thenon-disruptive resynchronization.
 14. The non-transitory machinereadable medium of claim 9, wherein the instructions cause the machineto: provide, by the second node, a second client device with access todata hosted by the second node during the non-disruptiveresynchronization.
 15. The non-transitory machine readable medium ofclaim 9, wherein the instructions cause the machine to: perform abaseline transfer using a base snapshot to transfer data of the firstconsistency group to the second node to create the second consistencygroup.
 16. The non-transitory machine readable medium of claim 9,wherein the instructions cause the machine to: provide the first clientdevice with access to the first consistency group while performing abaseline transfer using a base snapshot to transfer data of the firstconsistency group to the second node to create the second consistencygroup.
 17. A first node comprising: a memory storing instructions; and aprocessor coupled to the memory, the processor configured to execute theinstructions to cause the first node to: in response to receiving awrite request from a first client device when a first consistency grouphosted by the first node and a second consistency group hosted by asecond node are out of sync: execute the write request upon the firstconsistency group; and mark, within a dirty region log, a regionmodified by the write request as a dirty region; and perform anon-disruptive resynchronization to synchronize the second consistencygroup and the first consistency group, wherein the non-disruptiveresynchronization includes: committing incoming write requestscorresponding to the dirty regions to the first consistency group andnot the second consistency group and committing incoming write requestscorresponding to non-dirty regions to the first consistency group andthe second consistency group to place the first consistency group andsecond consistency group into a synchronous replication state.
 18. Thefirst node of claim 17, wherein the instructions cause the first nodeto: perform the non-disruptive resynchronization in response to acommunication loss.
 19. The first node of claim 17, wherein theinstructions cause the first node to: in response to performing athreshold number of asynchronous transfers from the first consistencygroup to the second consistency group, transfer data of the firstconsistency group to the second consistency group to synchronize thesecond consistency group and the first consistency group.
 20. The firstnode of claim 17, wherein the instructions cause the first node to:provide the first client device with access to the first consistencygroup during the non-disruptive resynchronization.